With the trend toward size and weight reduction in portable electronic devices, lithium secondary batteries have been increasingly used and developed as power supplies in these appliances. As for lithium secondary battery, however, excessively repeated charge and discharge, unfavorable short circuit and operation under high current for long time will release quantities of heat, and consequently, may lead to flammable components of electrolyte in a battery to burn, thermal breakdown or even explosion of a battery. Therefore, the safety of batteries are considered, especially when they are applied in electric automobiles. One critical approach to improve the safety of lithium secondary batteries is to enhance the thermal stability of materials for batteries.
By adding flame retarding components to a flammable organic electrolyte in a lithium ion battery, the thermal stability of electrolytic solution can be increased, thereby allowing the electrolytic solution to be flame retarding or nonflammable. As a result, battery burning or explosion is sufficiently prevented when the battery is worked under a condition of overheating. Organic phosphorus-based compounds are usually used as flame retarding components in electrolytes for lithium ion batteries. For example, alkyl phosphates such as trimethyl phosphate (TMP) and triethyl phosphate (TEP); phosphazenes such as hexamethyl phosphazene; compounds with phosphorus substituents; and compounds with phosphorus-nitrogen bonding can be mentioned and they are all excellent flame retardants. Nevertheless, most of the organic phosphorus-containing flame retardants have inferior electrochemical stability, so reductive degradation may easily occur on the surface of a carbonaceous negative electrode. Moreover, phosphorus-containing flame retardants have higher viscosity, resulting in adverse effects on the conductivity of electrolytes, such that the development of lithium-ion battery is restrained.
On the other hand, organic fluorinated compounds, i.e., hydrogen atoms thereof are substituted by fluorine atoms, are favorable to reduction in flammability of solvents for lithium ion batteries because of fewer hydrogen atoms contained on the solvent molecule. Fluorinated alkyl phosphates such as [tris-(2,2,2-trifluoroethyl)]phosphate (TFP) and [bis-(2,2,2-trifluoroethyl)]methylphosphate (BMP) have a retarding effect on the flammability and their electrochemical stability are improved by fluoro groups. However, these halogen-containing additives will produce toxic compounds when they are recollected and burned, thus harmful to environments.
Therefore, it is desired that there is a polymer electrolyte composition having properties of excellent conductivity and flame retardation as well as harmless to environments.
To overcome the above-mentioned problems, the present invention has been completed after the present inventors studied intensively and made improvement.